1. Field of the Invention
The present invention relates to scanning display optical systems which form images by scanning a light beam, and more specifically relates to an optical system suitable for use in an electronic view finder mounted in an imaging apparatus, such as a digital still camera and a video camera, or in an image display apparatus, such as a head mount display.
2. Description of the Related Art
Image display apparatuses which directly form an image on a viewer's retina by scanning a light beam emitted from a light-source unit in two-dimensional directions are suggested. For example, U.S. Pat. No. 5,467,104 discloses a retina scanning display which focuses a light beam scanned in two-dimensional directions on a primary imaging surface and forms a two-dimensional image on a viewer's retina through an ocular optical unit (eyepiece). In this apparatus, light must be scanned at a very high speed, and accordingly a very small device is used in a scanning unit, such as a mirror, for scanning light. Therefore, the light beam which is scanned is very thin, and the diameter of the light beam at the viewer's pupil is very small.
In order to increase the exit pupil diameter, U.S. Pat. No. 5,701,132 discloses a method of increasing the divergence angle of a light beam by arranging a light-diffusing member, such as a lens array and a diffuser, on a curved intermediate imaging surface formed by a scanned light beam and causing the light beam to pass through the light-diffusing member.
On the other hand, Japanese Patent Laid-Open Nos. 2001-4955 (paragraphs 0239 to 0241, FIG. 20, etc.) and 2001-194617 (paragraphs 0087 to 0104, FIG. 2, etc.) suggest optical systems including prisms which have rotationally asymmetric surfaces and which are made of materials with a refractive index n of 1 or more. Since the prisms are used, the sizes of the optical systems can be reduced by bending optical paths and the optical path lengths can be increased by a factor of n (n>1) with respect to the actual transmission distance. However, in these optical systems, intermediate imaging surfaces are disposed in the prisms, and therefore the light-diffusing member disclosed in U.S. Pat. No. 5,701,132 cannot be used.
Japanese Patent Laid-Open No. 2004-45496 (paragraphs 0087 to 0104, FIG. 2, etc.) (corresponding to U.S. Patent Application No. 2004-141221) suggests an optical system including a prism filled with a medium with a refractive index of 1 or more as an optical unit for focusing scanned light on a scanning plane. In this optical system, an image formed on the scanning surface is viewed through an ocular optical unit including a diffusing plate disposed on the scanning surface.
A combined lens or a prism disclosed in Japanese Patent Laid-Open No. 2004-45496 may be used as an optical unit for focusing a light beam on an intermediate imaging surface. However, when light enters a medium having different dispersions, chromatic aberrations are generated, and accordingly the image quality is reduced. In order to correct the chromatic aberrations in this optical unit, an additional correction optical unit is required. Accordingly, the size of the overall optical system increases.
In addition, when the light beam from the light-source unit is incident on the scanning member at a large angle, so-called scanning distortion occurs in the image formed on the scanning surface.
In addition, in order to form a two-dimensional image on the viewer's retina, an ocular optical unit for making the viewer's pupil and the exit pupil substantially coincide with each other is placed behind the scanning surface. Visibility of a displayed image can be improved by moving the ocular optical unit along an optical axis of the ocular optical unit. However, when the light beam is incident on the scanning surface from a direction largely shifted from a direction perpendicular to the scanning surface, the size of the viewed image varies as the ocular optical unit moves.
In addition, in the case in which an optical system includes a light-source unit with one or more light sources and a light-source optical unit having a beam combiner which combines light beams from the light sources into a single light beam, a rotationally symmetric optical unit can be used as the light-source optical unit to obtain high accuracy. However, when a plurality of light sources are arranged along a line or a plane and light beams from the light sources are individually incident on the scanning member, it is difficult to obtain a small light-source optical unit having characteristics suitable for all of the light sources and including only rotationally symmetric optical components.